It's not the Motown didn't like Mary, they just felt her contribution to the Supremes was more in image instead of talent. Mary had a deep singing voice much like Tina Turner or Anita Baker. In the 60s, it was the sweet higher pitched women who had the hit records. Connie Francis, Brenda Lee, Streisand, etc. Gladys Knight sang in the lower reaches and as you see her success was limited to only a couple hits now and then. She didn't explode on the scene until the mid 70s when music was changing.

I think what irked Berry during that period was that Mary snuck around and enjoyed her fame much moreso than the others. She got involved with the "party" crowd. Berry always advised the ladies to keep a distance from fans but Mary entertained them.

While she was partying Mary paid no attention into developing her voice into more of a lead singer. Therefore when Diana left, Motown did not want to invest in a group of Supremes with Mary as lead because her voice was not dependable, nor was it commercial. It wasn't until the mid 70s that Mary started taking vocal lessons and strengthened her voice. The years of heavy smoking and partying took a toll on her singing I'm afraid.

Then when Jean and Lynda left and Mary kept complaining to the media that she was being swept under the rug, Motown considered her a pain in the neck. Suzanne dePasse was once heard to say "the only thing Mary could do was look good." That summed it up. She was scenery for Diana.

Then when she wrote her books, Diana, Suzanne and Berry distanced themselves from her. She was now considered a loose cannon. Suzanne never forgave Mary for the Motown 25 debacle.